When I signed up for OSM about 8 years ago I checked the box "I consider my contribution public domain". I then saw what looks like my edits show up in google maps a few months later. (I have no idea if google maps imported OSM, or if they did an independent edit) I don't see such a box in my account settings now, so I'm guessing they removed it, but at one time at least it was legally possible.
Open street map's license allows anyone to use their data, but if you "build upon" their data then you must release your version with the same license. Google maps would not be able to incorporate OSM's licensed data to their own data without releasing their own proprietary data freely as well, which they aren't willing to do.
